In general, inkjet printing machines or printers include at least one printhead unit that ejects drops of liquid ink onto recording media or an imaging member for later transfer to media. Different types of ink may be used in inkjet printers. In one type of inkjet printer, phase change inks are used. Phase change inks remain in the solid phase at ambient temperature, but transition to a liquid phase when elevated to a melting temperature. The printhead unit ejects melted ink supplied to the unit onto media or an imaging member. Once the ink is ejected onto media, the ink droplets quickly solidify.
Phase change ink printers include one or more heaters that maintain a supply of phase change ink in a liquid state for use during printing operations. Some of the heaters maintain a supply of ink in the liquid state within reservoirs and other fluid conduits within the printheads. Typically, the heaters are electric heaters that consume electrical energy to maintain the phase change ink in a liquid phase. In order to reduce energy usage, phase change ink printers deactivate various components, including heaters, in the printer during a sleep mode to conserve energy. Loss of electrical energy during a sleep mode solidifies the ink held in the reservoirs and conduits.
The solidification of phase change ink within the printer presents issues for printing high quality documents when the printer emerges from sleep mode. As phase change ink within the printhead cools and solidifies, the ink contracts and air enters the reservoirs and fluid conduits within the printer. Reheating the solidified ink liquefies the ink and forms air bubbles in the liquefied ink. These air bubbles can prevent inkjets in the printhead from operating reliably. To eliminate air bubbles, a “purge” operation is performed. In a purge operation, pressure is applied to the reservoirs in the printheads to urge liquid ink and air bubbles through the nozzles of the inkjets in the printheads. The expelled ink flows down a face of the printhead and is collected in a waste ink receptacle. With the air bubbles removed from the melted ink, the inkjets are able to print ink drops reliably.
In existing printers, the purged ink is typically collected in a waste reservoir and is eventually discarded. Some printers have reclamation devices that reintroduce the waste ink into an ink supply instead of discarding the ink. In multi-color printers, however, the multiple colors of ink emitted during a purge operation often mix, and the resulting mixed ink is not suitable for direct reuse. In a printer using a common cyan, magenta, yellow, black (CMYK) color system, the mixed ink often appears to be dark brown or grey. The precise color of the mixed ink varies based on the types of ink used in the printer and on the proportional amounts of each ink that mix in the waste receptacle. Even in printers that do have separate waste reservoirs for various inks, the color quality of the individual inks may be reduced after a purge operation due to contaminants that are introduced into the purged ink.
One proposed ink reclamation apparatus pumps waste ink including one or more colors of ink into a black ink supply. Since the mixed ink colors have a darker color, the mixed waste ink and black ink mix together to form a color that approximates black closely enough for many print jobs. There are, however, limitations on the amount of mixed ink pumped into the black ink supply before the color of ink in the black ink supply deviates from the color of the pure black ink to a degree that negatively impacts image quality. Thus, much of the mixed waste ink cannot be recycled in existing printers without negatively affecting the image quality of printed images. Improvements to the printing process that enable greater reuse of purged ink in inkjet printers would be desirable.